This invention relates generally to electrochemical gas sensors for sensing gases which may be toxic to human beings in a specific area such as a mine shaft, for example. More particularly, this invention relates to a simplified calibration system for recalibrating such toxic gas sensors periodically in order to maintain them at a proper sensing level on a continuous basis. The invention may be utilized for recalibrating electrochemical sensors such as those taught and claimed in U.S. Pat. Nos. 3,776,832, 3,824,167 and 3,992,267, all by Harry G. Oswin et al and U.S. Pat. No. 4,001,103 to Blurton et al. Such sensors may be placed as noted above, in areas such as mine shafts or garages for detecting gases present in the ambient which may be dangerous to individuals in the area being protected by the sensor. Such gases include, for example, carbon monoxide, nitric oxide, nitrogen dioxide, and hydrazine.
Such gas sensors must be adjusted periodically because the sensor output changes with time, and two adjustments must be made, the "zero" calibration and the "span" calibration. The "zero" calibration consists of applying clean air to the sensing head of the toxic gas sensor in order to obtain an exact "zero" reading on the sensor. The "span" calibration consists of applying a gas of known concentration, and adjusting the gain of the resulting signal so that the system controller is displaying the correct gas concentration when it is actually sensed.
As will be appreciated, many such sensors are placed in remote areas. Since they must be periodically adjusted, in the past it has been necessary for two operators to go to the remote areas periodically for making the recalibration adjustments. As will be appreciated, the use of experienced technicians for making periodic adjustments in remote areas can be expensive.
With this invention, by contrast, an inexperienced person routinely present in the area being monitored may carry out the adjustments in a simplified manner simply by going to the arrangement of the invention here and making two adjustments. First by closing off the sensing head to ambient, so that the sensor is not exposed to the ambient for picking up any toxic gas, the zero adjustment may be made. At that point, the inexperienced operator may simply turn a knob until a light signal comes on indicating that a proper adjustment has been made. With the "span" adjustment, he simply connects a source of gas of known concentration supplied to him, and makes a second adjustment until a light signal comes on. It is not necessary for the inexperienced operator to have any training other than to note the appearance of the light signal. The arrangement, in accordance herewith, is such that if an over adjustment is made the light signal goes off. Thus, the arrangement provides for an upper and lower limit to adjust to a proper range of signal for both the "zero" and "span" adjustments.
Other objects and advantages of this invention will be apparent from the following description, the accompanying drawings, and the appended claims.